Typical mobile systems have two main operating modes: Idle mode and Connected mode. In Idle mode, the station (STA) characteristics include: no user service (i.e., no call or transaction in progress); monitoring of paging channels; available service request channels; 100% of the receiver is available for downlink measurements of the radio environment; background coordination; and unscheduled access point (AP) and/or technology reselection. In Connected mode, the STA characteristics include: an active user service (e.g., a call is in progress); handover is possible; limited receiver availability for measurements (since the user service takes priority); and fully coordinated, scheduled AP and/or technology handover.
Prior to entering Idle mode (e.g., at power-up), the STA must perform selection in order to determine the best AP and technology available for the requested user service. While in the Idle mode, the STA continuously examines neighboring APs and APs with different technologies. Upon determination of a “better” AP, the STA will transition over (i.e., perform “reselection”) to the new AP.
While in the Connected mode, a handover occurs upon transition from one AP to another AP offering “better” service, including switching to an AP using a different technology. In an ideal case, handover occurs without noticeable interruption of the active user service.
One goal is to achieve a seamless handover (i.e., to permit mobility of a STA) between different wireless network types, such as between different wireless local area network (WLAN) types or between a WLAN and a cellular system. Current technology does not provide for this type of handover.
FIG. 1 is a diagram of an existing cellular mobility model 100, showing a centralized radio resource management (RRM) approach to the mobility issue. A cellular STA 102 (e.g., a 2G mobile station or a 3G user equipment) is freely mobile among a plurality of APs 104. The APs 104 can include, but are not limited to, GSM base stations and FDD/CDMA Node Bs. The APs 104 are connected together via a radio network 106. A handover policy function (HPF) 108 is used to direct the handover of the STA 102 among the APs 104 as the STA 102 moves about. The HPF 108 is centrally located (e.g., in a 2G base station controller (BSC) or a 3G radio network controller (RNC)) and is connected to a network 110 (e.g., a switch or a server).
The HPF 108 provides coordination as the STA 102 moves about the different APs 104. The STA 102 sends measurements to the HPF 108, and the HPF 108 makes the final decision regarding handover and which AP 104 the STA 102 should be on.
In the model 100, semi-static frequency assignments are made to each AP 104 and some radio planning is required. In Idle mode, both intra-technology (e.g., GSM to GSM) and inter-technology (e.g., GSM to FDD/WCDMA) AP selection/reselection decisions are made in the STA 102 and are supported by system information (from the network 110) broadcast by the HPF 108. In Connected mode, AP handover decisions are made in the HPF 108 and are supported by measurements made by the STA 102 that are sent to the HPF 108 via L3 signaling.
FIG. 2 is a diagram of an existing WLAN mobility model 200, showing a distributed RRM approach to the mobility issue. An 802.x STA 202 is freely mobile among a plurality of APs 204, which can include, but are not limited to 802.11a and 802.16 APs. The APs 204 communicate via a radio network 206 and to a network 208 (e.g., a gateway or router).
In the model 200, dynamic frequency assignments are made to each AP 204 and radio planning is not required. The only type of handover supported in the mobility model 200 is an intra-technology (e.g., 802.11a to 802.11a) Idle mode handover, where the AP selection/reselection decision is made autonomously in the STA 202. The other handover types (Idle mode with inter-technology and Connected mode) are not supported in the mobility model 200.
In this distributed RRM approach, the APs 204 can be deployed anywhere and they dynamically manage themselves. There is no centralized point through which RRM is performed, and therefore, no element in the architecture to execute a handover.
FIG. 3 is a diagram of existing mobile system architectures for cellular and WLAN network types. A GPRS (2G) STA 300 includes a physical layer 302, a data link layer 304, and a network layer 306. The data link layer 304 includes a medium access control (MAC) sublayer 310 and a radio link control (RLC) sublayer 312. The network layer 306 includes a GSM radio resource (RR) manager 314, a mobility management (MM) protocol manager 316, and an Internet Protocol (IP)/convergence manager 318.
A 3GPP (3G) STA 320 includes a physical layer 322, a data link layer 324, and a network layer 326. The data link layer 324 includes a MAC sublayer 330 and a RLC sublayer 332. The network layer 326 includes a 3G RR controller 334, a MM protocol manager 336, and an IP/convergence manager 338.
An 802.xx STA 340 includes a physical layer 342, a data link layer 344, and a network layer 346. The data link layer 344 includes a MAC sublayer 350 and a logical link (LLC) sublayer 352. The network layer 346 includes a mobile IP manager 354 and an IP/convergence manager 356.
The RR manager/controller (314, 334) manages the instantaneous radio link, handling all of the information regarding a radio link. The MM protocol (316, 336, 354) handles network level issues, such as registration and location updating as the. STA moves about the system (i.e., issues outside of the call itself).
Current WLAN systems offer only a limited mobility capability. Intra-technology (e.g., 802.11 to 802.11) and inter-technology (e.g., 802.11 to 802.16) user transitions are supported using a “break before make” strategy that can be characterized as a reselection operation, as opposed to a handover operation in a typical full mobility system (e.g., GSM). This problem limits the growth of WLAN technologies, as this approach is unsatisfactory for supporting real time services such as voice and video streaming.